Raise an ArgumentError if required params missing

Until now, we've been returning empty resultset. There are two problems with
this:
  * for `search` it assumed the response was array, but this is being changed
  * for `advanced_search` it assumed the response was array, but it is always hash
  * it puts unnecessary knowledge of rummager into this gem

Alternatives to this change:
  * pass it to rummager and let it decide what to do (more network IO)
  * raise a custom error class (maybe inheriting from ArgumentError)
Merge pull request #68 from alphagov/cleaner_rummager_behaviour

Raise an ArgumentError if required params missing